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‎README.md

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| # | Title | Solution | Difficulty |
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| :-: | - | - | :-: |
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| <span id="1585">1585</span> | [Check If String Is Transformable With Substring Sort Operations](https://leetcode.com/problems/check-if-string-is-transformable-with-substring-sort-operations "检查字符串是否可以通过排序子字符串得到另一个字符串") | [Go](problems/check-if-string-is-transformable-with-substring-sort-operations) | Hard |
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| <span id="1584">1584</span> | [Min Cost to Connect All Points](https://leetcode.com/problems/min-cost-to-connect-all-points "连接所有点的最小费用") | [Go](problems/min-cost-to-connect-all-points) | Medium |
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| <span id="1583">1583</span> | [Count Unhappy Friends](https://leetcode.com/problems/count-unhappy-friends "统计不开心的朋友") | [Go](problems/count-unhappy-friends) | Medium |
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| <span id="1582">1582</span> | [Special Positions in a Binary Matrix](https://leetcode.com/problems/special-positions-in-a-binary-matrix "二进制矩阵中的特殊位置") | [Go](problems/special-positions-in-a-binary-matrix) | Easy |
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| <span id="1581">1581</span> | [Customer Who Visited but Did Not Make Any Transactions](https://leetcode.com/problems/customer-who-visited-but-did-not-make-any-transactions) 🔒 | [MySQL](problems/customer-who-visited-but-did-not-make-any-transactions) | Easy |
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| <span id="1580">1580</span> | [Put Boxes Into the Warehouse II](https://leetcode.com/problems/put-boxes-into-the-warehouse-ii) 🔒 | [Go](problems/put-boxes-into-the-warehouse-ii) | Medium |
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| <span id="1579">1579</span> | [Remove Max Number of Edges to Keep Graph Fully Traversable](https://leetcode.com/problems/remove-max-number-of-edges-to-keep-graph-fully-traversable "保证图可完全遍历") | [Go](problems/remove-max-number-of-edges-to-keep-graph-fully-traversable) | Hard |
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| <span id="1578">1578</span> | [Minimum Deletion Cost to Avoid Repeating Letters](https://leetcode.com/problems/minimum-deletion-cost-to-avoid-repeating-letters "避免重复字母的最小删除成本") | [Go](problems/minimum-deletion-cost-to-avoid-repeating-letters) | Medium |
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| <span id="1577">1577</span> | [Number of Ways Where Square of Number Is Equal to Product of Two Numbers](https://leetcode.com/problems/number-of-ways-where-square-of-number-is-equal-to-product-of-two-numbers "数的平方等于两数乘积的方法数") | [Go](problems/number-of-ways-where-square-of-number-is-equal-to-product-of-two-numbers) | Medium |
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<!--|This file generated by command(leetcode description); DO NOT EDIT. |-->
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<!--+----------------------------------------------------------------------+-->
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<!--|@author openset <openset.wang@gmail.com> |-->
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<!--|@link https://github.com/openset |-->
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<!--|@home https://github.com/openset/leetcode |-->
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<!--+----------------------------------------------------------------------+-->
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[< Previous](../min-cost-to-connect-all-points "Min Cost to Connect All Points")
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Next >
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## [1585. Check If String Is Transformable With Substring Sort Operations (Hard)](https://leetcode.com/problems/check-if-string-is-transformable-with-substring-sort-operations "检查字符串是否可以通过排序子字符串得到另一个字符串")
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<p>Given two strings&nbsp;<code>s</code> and <code>t</code>, you want to transform string&nbsp;<code>s</code> into string&nbsp;<code>t</code> using the following&nbsp;operation any number of times:</p>
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<ul>
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<li>Choose a <strong>non-empty</strong> substring in&nbsp;<code>s</code>&nbsp;and sort it in-place&nbsp;so the characters are in&nbsp;<strong>ascending order</strong>.</li>
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</ul>
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<p>For example, applying the operation on the underlined substring in&nbsp;<code>&quot;1<u>4234</u>&quot;</code>&nbsp;results in <code>&quot;1<u>2344</u>&quot;</code>.</p>
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<p>Return <code>true</code> if <em>it is possible to transform string <code>s</code>&nbsp;into string <code>t</code></em>. Otherwise,&nbsp;return <code>false</code>.</p>
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<p>A <strong>substring</strong>&nbsp;is a contiguous sequence of characters within a string.</p>
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<p>&nbsp;</p>
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<p><strong>Example 1:</strong></p>
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<pre>
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<strong>Input:</strong> s = &quot;84532&quot;, t = &quot;34852&quot;
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<strong>Output:</strong> true
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<strong>Explanation:</strong> You can transform s into t using the following sort operations:
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&quot;84<u>53</u>2&quot; (from index 2 to 3) -&gt; &quot;84<u>35</u>2&quot;
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&quot;<u>843</u>52&quot; (from index 0 to 2) -&gt; &quot;<u>348</u>52&quot;
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</pre>
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<p><strong>Example 2:</strong></p>
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<pre>
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<strong>Input:</strong> s = &quot;34521&quot;, t = &quot;23415&quot;
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<strong>Output:</strong> true
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<strong>Explanation:</strong> You can transform s into t using the following sort operations:
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&quot;<u>3452</u>1&quot; -&gt; &quot;<u>2345</u>1&quot;
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&quot;234<u>51</u>&quot; -&gt; &quot;234<u>15</u>&quot;
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</pre>
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<p><strong>Example 3:</strong></p>
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<pre>
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<strong>Input:</strong> s = &quot;12345&quot;, t = &quot;12435&quot;
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<strong>Output:</strong> false
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</pre>
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<p><strong>Example 4:</strong></p>
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<pre>
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<strong>Input:</strong> s = &quot;1&quot;, t = &quot;2&quot;
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<strong>Output:</strong> false
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</pre>
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<p>&nbsp;</p>
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<p><strong>Constraints:</strong></p>
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<ul>
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<li><code>s.length == t.length</code></li>
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<li><code>1 &lt;= s.length &lt;= 10<sup>5</sup></code></li>
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<li><code>s</code> and <code>t</code>&nbsp;only contain digits from <code>&#39;0&#39;</code> to <code>&#39;9&#39;</code>.</li>
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</ul>
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### Related Topics
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[[Greedy](../../tag/greedy/README.md)]
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[[String](../../tag/string/README.md)]
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### Hints
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<details>
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<summary>Hint 1</summary>
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Suppose the first digit you need is 'd'. How can you determine if it's possible to get that digit there?
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</details>
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<details>
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<summary>Hint 2</summary>
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Consider swapping adjacent characters to maintain relative ordering.
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</details>

‎problems/compare-version-numbers/README.md

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## [165. Compare Version Numbers (Medium)](https://leetcode.com/problems/compare-version-numbers "比较版本号")
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<p>Compare two version numbers <em>version1</em> and <em>version2</em>.<br />
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If <code><em>version1</em> &gt; <em>version2</em></code> return <code>1;</code>&nbsp;if <code><em>version1</em> &lt; <em>version2</em></code> return <code>-1;</code>otherwise return <code>0</code>.</p>
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<p>Given two version numbers,&nbsp;<code>version1</code> and <code>version2</code>, compare them.</p>
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<p>You may assume that the version strings are non-empty and contain only digits and the <code>.</code> character.</p>
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<p>The <code>.</code> character does not represent a decimal point and is used to separate number sequences.</p>
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<p>For instance, <code>2.5</code> is not &quot;two and a half&quot; or &quot;half way to version three&quot;, it is the fifth second-level revision of the second first-level revision.</p>
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<p>You may assume the default revision number for each level of a version number to be <code>0</code>. For example, version number <code>3.4</code> has a revision number of <code>3</code> and <code>4</code> for its first and second level revision number. Its third and fourth level revision number are both <code>0</code>.</p>
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<ul>
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</ul>
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<p>&nbsp;</p>
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<p>Version numbers consist of <strong>one or more revisions</strong> joined by a dot&nbsp;<code>&#39;.&#39;</code>. Each revision&nbsp;consists of <strong>digits</strong>&nbsp;and may contain leading <strong>zeros</strong>. Every revision contains <strong>at least one character</strong>. Revisions are <strong>0-indexed from left to right</strong>, with the leftmost revision being revision 0, the next revision being revision 1, and so on. For example&nbsp;<code>2.5.33</code>&nbsp;and&nbsp;<code>0.1</code>&nbsp;are valid version numbers.</p>
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<p>To compare version numbers, compare their revisions in <strong>left-to-right order</strong>. Revisions are compared using their&nbsp;<strong>integer value ignoring any leading zeros</strong>. This means that revisions&nbsp;<code>1</code>&nbsp;and&nbsp;<code>001</code>&nbsp;are considered&nbsp;<strong>equal</strong>. If a version number does not specify a revision at an index, then&nbsp;<strong>treat the revision as&nbsp;<code>0</code></strong>. For example, version&nbsp;<code>1.0</code> is less than version&nbsp;<code>1.1</code>&nbsp;because their revision 0s are the same, but their revision 1s are&nbsp;<code>0</code>&nbsp;and&nbsp;<code>1</code>&nbsp;respectively, and&nbsp;<code>0 &lt; 1</code>.</p>
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<p><em>Return the following:</em></p>
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<ul>
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<li>If <code>version1 &lt; version2</code>, return <code>-1</code>.</li>
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<li>If <code>version1 &gt; version2</code>, return <code>1</code>.</li>
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<li>Otherwise, return <code>0</code>.</li>
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</ul>
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<p>&nbsp;</p>
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<p><strong>Example 1:</strong></p>
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<pre>
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<strong>Input:</strong> <code><em>version1</em></code> = &quot;0.1&quot;, <code><em>version2</em></code> = &quot;1.1&quot;
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<strong>Output:</strong> -1</pre>
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<strong>Input:</strong> version1 = &quot;1.01&quot;, version2 = &quot;1.001&quot;
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<strong>Output:</strong> 0
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<strong>Explanation:</strong> Ignoring leading zeroes, both &quot;01&quot; and &quot;001&quot; represent the same integer &quot;1&quot;.
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</pre>
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<p><strong>Example 2:</strong></p>
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<pre>
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<strong>Input: </strong><code><em>version1</em></code> = &quot;1.0.1&quot;, <code><em>version2</em></code> = &quot;1&quot;
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<strong>Output:</strong> 1</pre>
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<strong>Input:</strong> version1 = &quot;1.0&quot;, version2 = &quot;1.0.0&quot;
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<strong>Output:</strong> 0
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<strong>Explanation:</strong> version1 does not specify revision 2, which means it is treated as &quot;0&quot;.
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</pre>
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<p><strong>Example 3:</strong></p>
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<pre>
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<strong>Input:</strong> <code><em>version1</em></code> = &quot;7.5.2.4&quot;, <code><em>version2</em></code> = &quot;7.5.3&quot;
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<strong>Output:</strong> -1</pre>
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<strong>Input:</strong> version1 = &quot;0.1&quot;, version2 = &quot;1.1&quot;
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<strong>Output:</strong> -1
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<strong>Explanation:</strong>&nbsp;version1&#39;s revision 0 is &quot;0&quot;, while version2&#39;s revision 0 is &quot;1&quot;. 0 &lt; 1, so version1 &lt; version2.
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</pre>
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<p><strong>Example 4:</strong></p>
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<pre>
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<strong>Input:</strong> <code><em>version1</em></code> = &quot;1.01&quot;, <code><em>version2</em></code> = &quot;1.001&quot;
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<strong>Output:</strong> 0
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<strong>Explanation:</strong> Ignoring leading zeroes, both "01" and "001" represent the same number "1"</pre>
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<strong>Input:</strong> version1 = &quot;1.0.1&quot;, version2 = &quot;1&quot;
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<strong>Output:</strong> 1
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</pre>
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<p><strong>Example 5:</strong></p>
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<pre>
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<strong>Input:</strong> <code><em>version1</em></code> = &quot;1.0&quot;, <code><em>version2</em></code> = &quot;1.0.0&quot;
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<strong>Output:</strong> 0
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<strong>Explanation:</strong> The first version number does not have a third level revision number, which means its third level revision number is default to "0"</pre>
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<strong>Input:</strong> version1 = &quot;7.5.2.4&quot;, version2 = &quot;7.5.3&quot;
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<strong>Output:</strong> -1
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</pre>
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<p>&nbsp;</p>
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<p><strong>Constraints:</strong></p>
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<p><strong>Note:</strong></p>
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<ol>
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<li>Version strings are composed of numeric strings separated by dots <code>.</code> and this numeric strings <strong>may</strong> have leading zeroes. </li>
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<li>Version strings do not start or end with dots, and they will not be two consecutive dots.</li>
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</ol>
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<ul>
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<li><code>1 &lt;= version1.length, version2.length &lt;= 500</code></li>
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<li><code>version1</code> and <code>version2</code>&nbsp;only contain digits and <code>&#39;.&#39;</code>.</li>
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<li><code>version1</code> and <code>version2</code>&nbsp;<strong>are valid version numbers</strong>.</li>
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<li>All the given revisions in&nbsp;<code>version1</code> and <code>version2</code>&nbsp;can be stored in&nbsp;a&nbsp;<strong>32-bit integer</strong>.</li>
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</ul>
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### Related Topics
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[[String](../../tag/string/README.md)]

‎problems/count-and-say/README.md

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### Similar Questions
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1. [Encode and Decode Strings](../encode-and-decode-strings) (Medium)
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1. [String Compression](../string-compression) (Easy)
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1. [String Compression](../string-compression) (Medium)
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### Hints
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<details>
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<!--|This file generated by command(leetcode description); DO NOT EDIT. |-->
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<!--+----------------------------------------------------------------------+-->
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<!--|@author openset <openset.wang@gmail.com> |-->
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<!--|@link https://github.com/openset |-->
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<!--|@home https://github.com/openset/leetcode |-->
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<!--+----------------------------------------------------------------------+-->
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[< Previous](../special-positions-in-a-binary-matrix "Special Positions in a Binary Matrix")
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[Next >](../min-cost-to-connect-all-points "Min Cost to Connect All Points")
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## [1583. Count Unhappy Friends (Medium)](https://leetcode.com/problems/count-unhappy-friends "统计不开心的朋友")
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<p>You are given a list of&nbsp;<code>preferences</code>&nbsp;for&nbsp;<code>n</code>&nbsp;friends, where <code>n</code> is always <strong>even</strong>.</p>
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<p>For each person <code>i</code>,&nbsp;<code>preferences[i]</code>&nbsp;contains&nbsp;a list of friends&nbsp;<strong>sorted</strong> in the <strong>order of preference</strong>. In other words, a friend earlier in the list is more preferred than a friend later in the list.&nbsp;Friends in&nbsp;each list are&nbsp;denoted by integers from <code>0</code> to <code>n-1</code>.</p>
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<p>All the friends are divided into pairs.&nbsp;The pairings are&nbsp;given in a list&nbsp;<code>pairs</code>,&nbsp;where <code>pairs[i] = [x<sub>i</sub>, y<sub>i</sub>]</code> denotes <code>x<sub>i</sub></code>&nbsp;is paired with <code>y<sub>i</sub></code> and <code>y<sub>i</sub></code> is paired with <code>x<sub>i</sub></code>.</p>
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<p>However, this pairing may cause some of the friends to be unhappy.&nbsp;A friend <code>x</code>&nbsp;is unhappy if <code>x</code>&nbsp;is paired with <code>y</code>&nbsp;and there exists a friend <code>u</code>&nbsp;who&nbsp;is paired with <code>v</code>&nbsp;but:</p>
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<ul>
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<li><code>x</code>&nbsp;prefers <code>u</code>&nbsp;over <code>y</code>,&nbsp;and</li>
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<li><code>u</code>&nbsp;prefers <code>x</code>&nbsp;over <code>v</code>.</li>
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</ul>
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<p>Return <em>the number of unhappy friends</em>.</p>
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<p>&nbsp;</p>
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<p><strong>Example 1:</strong></p>
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<pre>
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<strong>Input:</strong> n = 4, preferences = [[1, 2, 3], [3, 2, 0], [3, 1, 0], [1, 2, 0]], pairs = [[0, 1], [2, 3]]
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<strong>Output:</strong> 2
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<strong>Explanation:</strong>
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Friend 1 is unhappy because:
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- 1 is paired with 0 but prefers 3 over 0, and
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- 3 prefers 1 over 2.
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Friend 3 is unhappy because:
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- 3 is paired with 2 but prefers 1 over 2, and
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- 1 prefers 3 over 0.
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Friends 0 and 2 are happy.
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</pre>
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<p><strong>Example 2:</strong></p>
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<pre>
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<strong>Input:</strong> n = 2, preferences = [[1], [0]], pairs = [[1, 0]]
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<strong>Output:</strong> 0
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<strong>Explanation:</strong> Both friends 0 and 1 are happy.
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</pre>
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<p><strong>Example 3:</strong></p>
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<pre>
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<strong>Input:</strong> n = 4, preferences = [[1, 3, 2], [2, 3, 0], [1, 3, 0], [0, 2, 1]], pairs = [[1, 3], [0, 2]]
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<strong>Output:</strong> 4
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</pre>
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<p>&nbsp;</p>
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<p><strong>Constraints:</strong></p>
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<ul>
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<li><code>2 &lt;= n &lt;= 500</code></li>
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<li><code>n</code>&nbsp;is even.</li>
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<li><code>preferences.length&nbsp;== n</code></li>
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<li><code>preferences[i].length&nbsp;== n - 1</code></li>
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<li><code>0 &lt;= preferences[i][j] &lt;= n - 1</code></li>
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<li><code>preferences[i]</code>&nbsp;does not contain <code>i</code>.</li>
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<li>All values in&nbsp;<code>preferences[i]</code>&nbsp;are unique.</li>
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<li><code>pairs.length&nbsp;== n/2</code></li>
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<li><code>pairs[i].length&nbsp;== 2</code></li>
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<li><code>x<sub>i</sub> != y<sub>i</sub></code></li>
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<li><code>0 &lt;= x<sub>i</sub>, y<sub>i</sub>&nbsp;&lt;= n - 1</code></li>
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<li>Each person is contained in <strong>exactly one</strong> pair.</li>
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</ul>
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### Related Topics
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[[Array](../../tag/array/README.md)]
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### Hints
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<details>
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<summary>Hint 1</summary>
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Create a matrix "rank" where rank[i][j] holds how highly friend ‘i' views ‘j’. This allows for O(1) comparisons between people
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</details>
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<!--|This file generated by command(leetcode description); DO NOT EDIT. |-->
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<!--+----------------------------------------------------------------------+-->
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<!--|@author openset <openset.wang@gmail.com> |-->
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<!--|@link https://github.com/openset |-->
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<!--|@home https://github.com/openset/leetcode |-->
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<!--+----------------------------------------------------------------------+-->
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[< Previous](../put-boxes-into-the-warehouse-ii "Put Boxes Into the Warehouse II")
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[Next >](../special-positions-in-a-binary-matrix "Special Positions in a Binary Matrix")
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## [1581. Customer Who Visited but Did Not Make Any Transactions (Easy)](https://leetcode.com/problems/customer-who-visited-but-did-not-make-any-transactions "")
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Create table If Not Exists Visits(visit_id int, customer_id int);
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Create table If Not Exists Transactions(transaction_id int, visit_id int, amount int);
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Truncate table Visits;
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insert into Visits (visit_id, customer_id) values ('1', '23');
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insert into Visits (visit_id, customer_id) values ('2', '9');
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insert into Visits (visit_id, customer_id) values ('4', '30');
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insert into Visits (visit_id, customer_id) values ('5', '54');
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insert into Visits (visit_id, customer_id) values ('6', '96');
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insert into Visits (visit_id, customer_id) values ('7', '54');
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insert into Visits (visit_id, customer_id) values ('8', '54');
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Truncate table Transactions;
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insert into Transactions (transaction_id, visit_id, amount) values ('2', '5', '310');
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insert into Transactions (transaction_id, visit_id, amount) values ('3', '5', '300');
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insert into Transactions (transaction_id, visit_id, amount) values ('9', '5', '200');
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insert into Transactions (transaction_id, visit_id, amount) values ('12', '1', '910');
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insert into Transactions (transaction_id, visit_id, amount) values ('13', '2', '970');

‎problems/design-a-file-sharing-system/README.md

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### Related Topics
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[[Design](../../tag/design/README.md)]
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[[Array](../../tag/array/README.md)]
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### Hints

‎problems/design-compressed-string-iterator/README.md

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‎problems/encode-and-decode-strings/README.md

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@@ -61,5 +61,5 @@ vector&lt;string&gt; strs2 = decode(encoded_string);
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